The present invention relates to liquefied natural gas regasification. More particularly, the invention relates to offshore LNG regasification.
World natural gas consumption is rising faster than that of any other fossil fuel. About two-thirds of the increase in gas demand is in the industrial and power generation sectors, while the remaining one-third is in space heating of buildings and homes. Recent technological improvements in the design, efficiency, and operation of combined cycle gas turbines have tilted the economics of power generation in favor of natural gas. With the demand for electricity constantly increasing, the demand for natural gas can be expected to increase even further.
As is the case with oil, natural gas is unevenly distributed throughout the world. More than one-third of the world""s gas reserves are in the territory of the Former Soviet Union. The second largest gas reserve is located in the Middle East. However, North America accounts for more than one-half of the world""s gas consumption. The United States alone consumes about 2.4 tcf more natural gas per year than it produces. Germany and Japan also import large amounts of natural gas each year. Thus, natural gas frequently needs to be transported from its production locations to the consumption locations. However, the low density of natural gas makes it more expensive to transport than oil. A section of pipe in oil service can hold 15 times more energy than when used to transport high pressure gas. An alternative method of natural gas transportation is by ships. While natural gas can be piped in a gaseous state, it needs to be liquefied so that it may be economically transported by ships. When natural gas is supercooled to minus 162xc2x0 C., it becomes liquid, and takes up only {fraction (1/600)}th as much space as gas. Liquefaction makes it practical to ship natural gas in large volumes, using specially designed ships that maintain the cargo""s ultra-low temperature. Once liquefied natural gas (LNG) is transported to its destination, it is converted into gas at a regasification terminal before it is sent to the consuming end. Thus, regasification terminals are important links in the natural gas supply chain.
Most regasification units in operation are located at onshore terminals. FIG. 1 illustrates one such terminal. As shown in FIG. 1, an onshore LNG regasification terminal typically consists of a pier or jetty 1, storage tanks 2, and regasification plants 3. An LNG ship 5 comes and berths at the pier 1, and off-loads its cargo of LNG to storage tanks 2 which keep the gas in the same liquid state as they are transported. LNG in the storage tank 2 is later regasified at the regasification plant 3 to produce natural gas which is then transferred to end users through pipelines (not shown).
The storage tanks 2 typically are double barrier tanks with an xe2x80x9cinteriorxe2x80x9d container installed inside an independently reinforced concrete caisson. Built of concrete and steel, the inner tanks typically are made of 9% nickel steel and the secondary containers are typically made of pre-stressed concrete with a steel liner. The regasification plant 3 (or regasification unit) typically consists of heat exchangers (vaporizers) 3a, pumps 3b, and compressors 3c. Regasifying or regasification means bringing the cold LNG to the gaseous state at the ambient temperature and proper pressure so that it can be exported and fed into the existing pipeline grid for sale and transport to the consuming end.
To date, most LNG regasification facilities have been built onshore. However, public concern about safety has caused the gas industry to look for remote sites for such facilities. One alternative is to build the regasifi cation facility offshore. Various offshore terminals with different configurations and combinations have been proposed. Most of these offshore designs are based on large floating barges installed to mooring systems. As shown in FIG. 2, an offshore regasification terminal typically includes a barge 16 with storage tanks 11 and means (not shown) for a vessel to approach, berth and offload its cargo. The barge 16 includes at least one regasification unit 12 and a connector 13 that is adapted to connect to an underwater pipeline 15 via a riser 14. Offshore LNG regasification terminals offer potential advantages over their onshore counterparts because they are further removed from populated areas thus minimizing risk to neighboring areas and reducing ship traffic and minimizing ships traveling in restricted waterways.
In an offshore terminal, the storage tanks 11 are incorporated in a barge 16 that supports the tanks. The storage tanks 11 may be membrane or non-membrane (freestanding) tanks. The main difference between these two types of tanks is how they are insulated. Membrane tanks are typically made with an inner liner of, for example, stainless steel or a specialized alloy such as invar (35% nickel steel). Non-membrane (freestanding) tanks are either spherical or prismatic and are typically made of aluminum or 9% nickel steel. In membrane tanks, insulation is built outside the liner in a manner that allows circulation of an inert gas, usually liquid nitrogen, through the insulating material, in order to monitor the integrity of the barrier. In non-membrane tanks, whether spherical or prismatic, the insulation is built and applied to the outside surface of the tanks.
Both types of tanks, whether prismatic or spherical, and whether membrane insulated or not, have been proposed for use in offshore LNG regasification systems. However, prismatic tanks are preferable, because as in the ships they allow for a more rational use of the space available in the offshore barge. As is the case for onshore terminals, in order to export the gas into the pipeline system, the cold-stored LNG must be brought to ambient temperature and the corresponding pipeline pressure. This is accomplished at the regasification unit 12 fitted onboard the barge. The regasification unit 12 is usually built on top of the tanks 11, in case of prismatic tanks, or around and between them, in case of spherical tanks (not shown).
The flow of gas from the barge 16 to the onshore pipeline system (not shown) may be accomplished through a riser 14 connected to the sea bottom where an underwater pipeline 15 receiving end exists. The riser 14 connection at the barge end may be made through a fixed point in the case where the barge 16 is spread-moored, with mooring lines directly attached to several points on the barge 16. The riser 14 connection may also be through a turret system such as shown at 13, that provides a common end for the moored lines 17, and connects the riser 14 through a swivel (not shown), so that the barge 16 may weathervane due to change of direction of the environmental conditions while gas is flowing to the riser 14. Instead of the turret system 13, the barge 16 may also be moored to a CALM buoy (not shown), that also provides single point mooring, and thus weathervaning, with the mooring system attached to buoy itself and thus independent of the barge 16. The preferred solution is for the barge to 16 weathervane through a connector such as a turret or CALM buoy system. This scheme allows the ships carrying LNG to approach and moor alongside the barge 16 thus allowing side-byside offloading the LNG cargo from the ships; side-by-side offloading is more convenient. However, in order to conveniently and safely moor the LNG ships alongside the barge 16, the barge 16 has to be longer than any conventional LNG carrier.
U.S. Pat. No. 6,089,022, issued to Zednik et al., discloses a method to regasify LNG onboard an LNG tanker before transferring the gas to an onshore facility. This approach requires that each LNG tanker be equipped with a vaporizer. A specially designed FSRU (floating LNG storage and regasification unit) has also been designed based on a tanker type double-hulled vessel permanently moored in water by means of a turret single point mooring (SPM).
While these offshore regasification approaches offer some advantages over onshore facilities, it is desirable that an offshore regasification system permits regular LNG carriers to unload their cargoes and to regasify LNG before it is transported to an onshore facility.
One aspect of the invention is an offshore liquefied natural gas (LNG) regasification system, which includes a mobile floating platform having a regasification unit disposed on it. The regasification unit is adapted to operatively couple to an outlet of a liquefied natural gas carrier. The regasification unit is adapted to operatively couple at its outlet to a tap on an offshore gas pipeline. The mobile floating platform is adapted to moor at least one liquefied natural gas carrier. In one embodiment, the platform is a modified very large crude carrier (VLCC). In one embodiment, the VLCC includes a propulsion unit, so that the VLCC may sail to a location for regasifying LNG according to market demand. In one embodiment, the floating platform includes a system for maintaining freeboard substantially the same as the freeboard of an LNG carrier berthed to the floating platform as the LNG is offloaded and regasified.
Another aspect of the invention relates to methods for regasifying LNG at a selected location. One embodiment of the invention comprises determining the selected location based on market demand for natural gas; moving an offshore regasification system to the selected location, berthing an LNG carrier next to the regasification system, and offloading and regasifying the liquefied natural gas.
Other aspects of the invention will become apparent form the following discussion.